Matrix-assisted laser ionization and desorption time-of-flight mass spectrometry is a recently developed technique which is particularly useful for the sensitive analysis of large biomolecules. The matrix is a material that assists in the transfer of energy to the analyte molecule, allowing it to be ionized without significant fragmentation, and leave the surface of a target that is being irradiated with a laser. Typically, a few microliters of a solution containing sample molecules at concentrations of about 1 .mu.g/.mu.L are mixed with 10-20 .mu.L of a solution containing matrix molecules at concentrations of about 10 .mu.g/.mu.L. A few microliters of this mixture are then deposited on a suitable substrate and dried in air.
As the sample dries, crystals of matrix are formed and the sample is thought to be incorporated into the crystal. The substrate is then introduced through a vacuum lock into a time-of-flight mass spectrometer system. In such systems, a high voltage source (often 30 KV or more) will be connected to the substrate.
Once the sample has been introduced into the mass spectrometer, a pulsed laser is used to irradiate the sample on the substrate. The interaction of the laser radiation with the matrix molecules leads, by a process that is only partly understood today, to the formation and desorption of largely intact, ionized sample molecules. Predominantly these ions are of a type known as (M+H)+ ions, that is, the neutral sample molecule (M) is ionized by the attachment of a proton. This ionization process has some similarity to the process called "chemical ionization" used conventionally in gas chromatography/mass spectrometry.
Most frequently these ions are analyzed in so-called linear time-of-flight (TOF) mass spectrometers. The ions, once formed, are accelerated by an electric field and then allowed to travel in straight lines until they are detected. The transit time between ion formation and detection can be used to determine the mass of the species from which the ions are generated. Typical linear TOF systems are described in U.S. Pat. No. 5,045,694 (Beavis and Chait). Such linear devices provide only modest mass resolving power, e.g. 50-800, because they are unable to compensate for various known aberrations. A dominant aberration in such linear systems stems from the fact that the ions are formed with a wide distribution of initial velocities. This means that for an ion of a given mass there will be a distribution of arrival times at the detector that will limit the mass resolving power of such a device, since ions with more initial velocity in the forward direction will arrive sooner than ions with less initial velocity.
Techniques for compensating for such aberrations resulting from the initial velocity distribution in TOF mass spectrometers are well-known. The primary technique is to provide an electrostatic mirror, called a reflectron, which reverses the direction of travel of the ions in such a way that the effects of these initial velocity distributions on ion transit times are eliminated. A recent review article describing such devices is "Time-of-flight Mass Spectrometry: An increasing Role in the Life Sciences", R. J. Cotter, Biomed. Env. Mass Spectrom., 18: 513-532 (1989). The practice of matrix-assisted laser desorption and ionization in reflectron-based instruments is also known and typically produces mass resolutions of 2000-4000 for molecules less than 5000 Daltons in molecular weight.
In both linear and reflectron-based TOF instruments, it is thought that a significant factor limiting mass resolution is the interaction of sample ions with other desorbed matrix ions and molecules i.e., as the desorbed biomolecules leave the surface of the target, they may encounter a plume of matrix ions and molecules. Interactions with this plume may change the energy of desorbed biomolecule ions, but not in a homogeneous manner. Some biomolecules may gain more kinetic energy, some may lose kinetic energy. Thus, the time of arrival of the biomolecules is not exactly the same because some are flying faster, and some slower, than the mean. The net result is band broadening and a concomitant loss in mass resolution. To date, the inventors are not aware of efforts to narrow the initial velocity distribution by manipulation of the matrix. Crystal formation occurs as the sample/matrix mixture dries down in a largely uncontrolled manner. It is postulated that this leads to variability in the analysis of the sample. In addition, the necessity of mixing sample with matrix prior to dry down often results in inefficient use of sample. Accordingly, it would be desirable to provide methods and apparatus for providing greater consistency in the sample preparation process and more efficient use of sample.
Hillenkamp et al., in British Patent Nos. GB 2236185A, and GB 2236186A, disclose surfaces and matrices for laser desorption of biomolecules from surfaces. GB 2236185A discloses a two-dimensional layer comprising a matrix ("absorbing component") underlying the substrate. The application is aimed at macromolecular blotting. GB 2236186A discloses desorption of biomolecules using 337 nm or higher laser radiation on a similar surface. Sinapinic acid is shown as the matrix.
Cottrell, PCT/GB90/00973, discloses a method for preparing a sample for analysis by LDMS that includes electrospraying the matrix (nicotinic acid) onto a target surface, then sample in TFA is applied and dried down. Finally, the sample is introduced into the mass spectrometer and a laser is directed onto the sample, desorbing the sample.
Cottrell, PCT/GB90/00974, discloses a method for preparing a sample for analysis by LDMS that includes electrospraying a substrate (nitrocellulose) onto a target, depositing a sample dissolved in aqueous 0.1% trifluoroacetic acid (TFA), and then drying it down. Matrix material (nicotinic acid, 30 mM in acetone) is then applied in droplet form to cover the dried-down sample and dissolve the substrate nitrocellulose. Sample, substrate and matrix dry down together in an intermixed form. If the sample is a protein, the protein adsorbs to the nitrocellulose by hydrophobic interactions. Loss of mass resolution is caused when excess matrix is evaporated from the surface of the target, causing the plasma effect described supra.
Cottrell, PCT/GB90/00975, additionally discloses the use of various matrix materials such as cinnamic acid, benzoic acid, or coumarin in the methods disclosed above.
Beavis, J. Phys. D: Appl. Phys., 26(3), 442-7, has emphasized the desirability of crystal formation.
Hutchens (Proceedings of the 41st American Society of Mass Spectrometry Conference on Mass Spectrometry and Allied Topics, May 31-Jun. 4, 1993, pp. 781a-781b) has described a technique called Surface-Enhanced Neat Desorption (SEND), in which energy-absorbing molecules are covalently bonded to an inert substrate, allegedly providing a matrix-free method for introducing large molecular weight biopolymers into the gas phase without fragmentation. However, this technique appears to have limited reproducibility.
Other efforts to laser desorb biomolecules include those of Tanaka et al. (Rapid Commun. Mass Spectrom. 2, 151-153, (1988)) who describe a system for matrix-assisted laser desorption and ionization in which the sample is dissolved in glycerol containing small cobalt particles. Cornett et al, Anal. Chem. 65: 2608-2613 (1993) has described a system in which various energy absorbing molecules such as rhodamine 6G are dissolved along with the sample molecules in a liquid matrix such as 3-nitrobenzyl alcohol.
Williams in U.S. Pat. No. 5,135,870 and Becker in U.S. Pat. No. 4,920,264 describe systems involving frozen layers of ice for the desorption and ionization of DNA. None of these systems have demonstrated surprising resolution to date.
Thus the need exists for a laser desorpt on matrix composition that can decrease the loss in mass resolution that occurs when using prior art procedures as well as provide increased sensitivity.